The present invention relates to processing of pumpable food products, and more particularly to processing systems and methods for deactivating organisms in pumpable food products or foodstuffs, which systems and methods extend the shelf life of such food products or foodstuffs. Even more particularly, the present invention relates to deactivating organisms or pumpable food products or foodstuffs in a high strength electric field system treatment employing serial electrodes separated by an insulating section.
As used herein the phrases "deactivating organisms," "deactivate organisms," "deactivation of organisms" and similar phrases refer to the killing or sterilization of living organisms such as bacteria, viruses, fungi, protozoa, parasites and the like.
Substantial technical effort has been directed to the preservation of perishable fluid food products such as milk products, natural fruit juices, liquid egg products, and pumpable meat products, such as ground beef or turkey. Such liquid food products may normally contain a wide variety of microorganisms, and are excellent culture media for such microorganisms.
Practical preservation methods which have found significant commercial application predominantly utilize heat treatment such as pasteurization to inactivate or reduce microorganism population. For example, milk products are conventionally pasteurized at a minimum temperature of at least about 72.degree. C. for 15 seconds (or equivalent time/temperature relationship) to destroy pathogenic bacteria and most of the nonpathogenic organisms, with degradative enzyme systems also being partially or totally inactivated. However, products processed in this manner are still generally unsterile and have limited shelf life, even at refrigeration temperature. The shelf life of liquid foodstuffs may be substantially extended by higher heat treatment processes such as "ultra high pasteurization", or "ultra high temperature" ("UHT") treatment, at a temperature of 140.degree. C. for four seconds. These processes are used in conjunction with aseptic packaging to achieve complete destruction of all bacteria and spores within the food product, however, such heat treatment typically adversely affects the flavor of the food product, at least partially denatures its protein content or otherwise adversely affects desired properties of the fluid food product. Other approaches to liquid food preservation, which also have certain disadvantages, include the use of chemical additives or ionizing radiation.
The bactericidal effects of electric currents have also been investigated since the end of the 19th century, with various efforts having been made to utilize electrical currents for treating food products. Such efforts are described in U.S. Pat. Nos. 1,900,509, 2,428,328, 2,428,329 and 4,457,221 and German Patents 1,946,267 and 2,907,887, inter alia, all of which are incorporated herein by reference. The lethal effects of low-frequency alternating current with low electric field strength have been largely attributed to the formation of electrolytic chemical products from the application of current through direct contact electrodes, as well as ohmic heating produced by current flow through an electrically resistive medium. Unfortunately however, the electrolytic chemical products generated by low frequency, low strength electric field methods may be undesirable in fluid foodstuffs, and heating, as noted above, may also cause undesirable effects in the fluid foodstuffs.
As described in U.S. Pat. No. 3,594,115, incorporated herein by reference, lethal effects of high voltage arc discharges have also been attributed to electrohydraulic shock waves. The utilization of explosive arc discharges to produce microbiologically lethal shock waves has not found wide-spread application as it is not a very effective means for preserving edible liquid foodstuffs. In addition, such explosive arc discharges can produce undesirable chemical byproducts in the foodstuffs being treated.
More recently, the effect of strong electric fields (or very high strength electric fields) on microorganisms has been studied as a mechanism for reversibly or irreversibly increasing the permeability of the cell membrane of microorganisms and individual cells. The application of very high strength electric fields to reversibly increase the permeability of cells has been used to carry out cell fusion of living cells and to introduce normally excluded components into living cells. Very high strength electric fields in non-nutrient media can also have a direct irreversible lethal effect upon microorganisms with the rate of deactivation dependent upon the field strength above a critical field level and the duration of the applied very high strength electric field.
A pulsed field treatment apparatus, which uses very high strength electric field pulses of very short duration, to deactivate microorganisms in food products is shown in U.S. Pat. No. 5,235,905 (the '905 patent); and U.S. Pat. No. 5,048,404 (the '404 patent), issued to Bushnell et al., and U.S. Pat. No. 4,838,154 (the '154 patent); and U.S. Pat. No. 4,695,472 (the '472 patent), issued to Dunn et al., all of which are incorporated herein by reference. The prevention of electro-phoretic and electro-chemical effects in these apparatuses is described in U.S. Pat. Nos. 5,393,541 and 5,447,733, issued to Bushnell, et al. (the '541 patent and the '733 patent), both of which are incorporated herein by reference. Generally, in accordance with the these patents, methods and apparatuses are provided for preserving fluid foodstuffs (or pumpable foodstuffs), which are normally excellent bacteriological growth media. Such preservation is achieved by applying very high strength electric field pulses (of at least 5000 v/cm) of very short duration (of no more than about 100 microseconds) through all of the pumpable foodstuff.
By "pumpable," "liquid," or "fluid" "food product" or "foodstuff" is meant an edible, food product having a viscosity or extrusion capacity such that the food product may be forced to flow through a treatment zone, e.g., less than about 1000 poise. The products include extrudable products, such as doughs or meat emulsions such as hamburger; fluid products such as beverages, gravies, sauces, soups, and fluid dairy products such as milk; food-particulate containing food slurries such as stews; food-particulate containing soups, and cooked or uncooked vegetable or grain slurries; and gelatinous foods such as eggs and gelatins.
By "bacteriological growth medium" is meant that upon storage at a temperature in the range of 0.degree. C. to about 30.degree. C., the fluid foodstuff, with its indigenous microbiological population or when seeded with test organisms, will demonstrate an increase in biological content or activity as a function of time as detectable by direct microscopic counts, colony forming units on appropriate secondary media, metabolic end product analyses, biological dry or wet weight or other qualitative or quantitative analytical methodology for monitoring increase in biological activity or content. For example, under such conditions the microbiological population of a pumpable foodstuff which is a bacteriological growth medium may at least double over a time period of two days.
The compositions of typical fluid food products which are biological growth media, derived from "Nutritive Value of American Foods in Common Units", Agriculture Handbook No. 456 of the U.S. Department of Agriculture (1975), are as follows:
______________________________________ FLUID FOODSTUFFS Fluid Carbo- Food Water Protein Fat hydrate Na K Product Wt % Wt % Wt % Wt % Wt % Wt % ______________________________________ Whole Milk 87.4 3.48 3.48 4.91 .05 .144 (3.5% fat) Yogurt** 89.0 3.40 1.68 5.22 .050 .142 Raw Orange 88.3 .685 .20 10.0 .0008 .2 Juice Grape Juice 82.9 .001 tr. .166 .0019 .115 Raw Lemon 91.0 .41 .20 8.0 .0008 .14 Juice Raw Grape- 90.0 .48 .08 9.18 .0008 .16 Fruit Juice Apple Juice 87.8 .08 tr. 11.9 .0008 .10 Raw Whole 73.7 12.88 11.50 .90 .12 .13 Eggs Fresh Egg 87.6 10.88 .02 .79 .15 .14 Whites Split Pea 70.7 6.99 2.60 16.99 .77 .22 Soup* Tomato 81.0 1.60 2.10 12.69 .79 .187 Soup* Tomato 68.6 2.0 .588 25.4 1.04 .362 Catsup Vegetable 91.9 2.08 .898 3.9 .427 .066 beef soup ______________________________________ *condensed commercial **from partially skimmed milk
Very high strength electric fields may be applied by means of treatment cells of high-field-strength design, examples of which are described in detail by Bushnell et al. and Dunn et al. Basically, the foodstuff is, in practice, electrically interposed between a first electrode, and a second electrode. The very high strength electric field is generated between the first and second electrodes such that the very high strength electric field passes through the foodstuff, subjecting any microorganisms therein to the very high strength electric field. Generally, the second electrode consists of a grounded electrode, and a relatively higher or lower voltage potential is applied to the first electrode.
In the Bushnell et al. patents and the Dunn et al. patents, the pumpable fluid foodstuff is subjected to at least one very high strength electric field and current density electrical pulse, and at least a portion of the fluid foodstuff is subjected to a plurality of very high strength electric field and current density pulses, in a high-strength electric pulse treatment zone. In one processing technique, the liquid foodstuff is introduced into a treatment zone, or cell, between two coaxial electrodes which have a parallel configuration adapted to produce a substantially uniform electric field thereinbetween without dielectric tracking or other breakdown. By "parallel" configuration it is meant that food product passes between the electrodes, such that electric flux lines are approximately normal to direction of flow. (By "serial" configuration, in contrast, it is meant that food product passes a first electrode then a second electrode, such that electric flux lines are generally parallel to the direction of flow.). Using these parallel-configured electrodes, very high strength electric field pulses are applied to the electrodes to subject the liquid foodstuff to multiple pulse treatment by the pulsed field apparatus. In order to generate the very high strength electric field pulses, the pulsed field apparatus employs, for example, a lumped transmission line circuit, a Blumlein transmission circuit and/or a capacitive discharge circuit. Alternatively, the Bushnell et al. patents describe the use of field reversal techniques in capacitive discharge systems (or pulse forming networks) to increase the effective potential across the treatment cell. For example, by applying a short electric field pulse of very high electric field strength (e.g., 20,000 volts per centimeter) across a treatment cell for a short period of time (e.g., 2 microseconds) of one polarity, followed by abrupt reversal of the applied potential within a short time period (e.g., 2 microseconds), an effective field approaching 40 kilovolts per centimeter is achieved across the cell.
If liquid foodstuff (i.e., pumpable foodstuff) is continuously introduced into the treatment zone to which very high strength electric field pulses are periodically applied, and fluid foodstuff is concomitantly withdrawn from the treatment zone, the rate of passage of the liquid foodstuff through the treatment zone can be coordinated with the pulse treatment rate so that all of the pumpable foodstuff is subjected to at least one very high strength electric field pulse within the treatment zone. The liquid foodstuff may be subjected to treatment in a sequential plurality of such treatment zones, or cells, as is described in more detail by Bushnell et al.